The S&P 500 index finally closed 10 percent beneath its record high Monday, breaking its 46-month streak of no 10-percent corrections.

That was the fifth-longest correction-free streak ever. "And if history is any guide, that means there is a good chance that a 20 percent bear-market selloff is coming," writes Tomi Kilgore of MarketWatch.

The S&P 500 closed at 1,893.21 Monday 11.3 percent below its May 20 record of 2,134.72. The index stood at 1,937 Tuesday afternoon.

Of the four other correction-free periods longer than 36 months, three ended with declines of more than 20 percent, Gail Dudack, chief investment strategist at Dudack Research Group, told MarketWatch. Those periods encompassed 1966, 1987, 1997 and 2007.

Only the 1997 drop, sparked by the devaluation of the Thai baht, failed to reach 20 percent, bottoming out at 10.8 percent.

Federal Reserve tightening ended the party in the other three instances. The 2007 decline presaged the financial crisis of 2008. On Oct. 19, 1987, the Dow Jones Industrial Average fell 23 percent.

Meanwhile, whether it's now or later, the 6 ½-year bull market for U.S. stocks is headed for an end, says London Telegraph columnist Matthew Lynn. And what are the ramifications?

First, "we can forget a rise in interest rates," he writes. "The markets have already worked out that the talk of the Federal Reserve pushing rates up in September can now be safely ignored."

The Fed has kept short-term interest rates at a record low just above zero since December 2008. Fed officials have been saying for weeks that they will raise rates by year-end. And until the stock market dive began a week ago, many economists expected a move in September.

The S&P 500 index plunged 6.2 percent from Aug. 17 to Aug. 24 before rebounding 2.3 percent Tuesday.

"Second, we will see more stimulus," Lynn says. He notes that the Fed funded the financial system with liquidity after the stock market crashes of 1987, 2000 and 2008-09. "Why would we expect it to be any different this time around?" Lynn asks.

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